In recent years, there has been utilized a magnetron plasma etching apparatus for performing an etching in a microprocessing by producing a high density plasma at a relatively low pressure atmosphere. In this apparatus, an RF (high frequency) electric field is formed inside a processing space such that electric lines of force vertically penetrate (i.e., an electric field direction is perpendicular to) a semiconductor wafer. In the specification, the term “vertical direction” refers to a direction of gravity. Further, in the processing space, a magnetic field is formed by a permanent magnet such that magnetic force lines are normal to the electric lines of force (i.e., a magnetic field direction is horizontal). By such orthogonal electric and magnetic fields, a magnetron discharge accompanied by drift motions of electrons is performed, so that an etching can be carried out with very high efficiency.
As an example of a magnet for use in the magnetron plasma etching apparatus, there is a dipole ring magnet. The dipole ring magnet has a multiplicity of columnar anisotropic magnet segments disposed in a ring shape around a processing chamber. Magnetization directions of these magnet segments are shifted slightly with respect to each other, such that a uniform horizontal magnetic field is formed on the whole.
As another example of the magnet for use in the magnetron plasma etching apparatus, there is a multi-dipole ring magnet. The multi-dipole ring magnet has a multiplicity of magnet segments disposed in a ring shape to surround a wafer such that N poles and S poles thereof are alternately arranged. The multi-dipole ring magnet forms a multi-pole magnetic field surrounding the circumference of the wafer without forming a magnetic field on an upper surface of the wafer. The above-described dipole magnetic field and multi-pole magnetic field are selectively used depending on a process.
In a plasma processing apparatus including but not limited to the magnetron plasma etching apparatus, it is necessary to prevent a plasma from arriving at a lower part of the processing chamber and thus causing an abnormal discharge. For this, an annular baffle plate is installed at a downward position of the wafer between a mounting table for mounting thereon the wafer and a processing chamber wall to shut off the plasma. In other words, the baffle plate is interposed between the processing space and a gas exhaust port to confine the plasma inside the processing space. The baffle plate has multiple through holes allowing the processing space and the gas exhaust port to communicate with each other.
However, as will be explained below, an abnormal discharge or a plasma leak toward a downside of the baffle plate disposed as mentioned above is observed in a conventional magnetron plasma etching apparatus, according to a study of the present inventors.